Liquefied atmospheric gases, including nitrogen, oxygen and argon, are finding increasing uses in industry. Such liquefied atmospheric gases provide cryogenic capabilities for various industrial processes, are more economical to transport in merchant supply and provide ready and economical sources of gaseous product from liquid storage facilities. For instance, liquid nitrogen is increasingly used to freeze food products, to cryogenically embrittle used materials for cleaning or recycle, and as a supply of gaseous nitrogen inerting medium for various industrial processes.
The conventional process for making large quantities of liquid nitrogen and/or liquid oxygen from an air feed is to include an expander scheme with the conventional multiple column distillation system. The expander scheme provides at least a portion of the large amount of refrigeration that is required to remove a large percentage of the air feed as liquid product vis-a-vis a small percentage of the air feed or no percentage of the air feed as liquid product. (As used herein, a "large percentage" of the air feed is defined as at least 15% of the air feed). This inclusion of an expander scheme with the conventional multiple column distillation system is generally referred to in the industry as a liquefier and that is how the term liquefier is used herein.
The most common liquefier probably falls into the category of nitrogen recycle liquefiers. In a nitrogen recycle liquefier, the expander scheme is integrated with the recycling of low pressure column nitrogen overhead such as taught in U.S. Pat. Nos. 3,605,422 and 4,894,076. The nitrogen recycle liquefiers, no matter how many expanders there are, do not try to use the feed air for generating refrigeration before it is fed into the distillation column systems.
U.S. Pat. No. 4,152,130 introduces the concept of air recycling. In the air recycle liquefiers, a major fraction of the air streams entering cold box are compressed to pressures higher than that needed for the distillation system. At least a portion of the high pressure air is isentropically expanded to provide the refrigeration needed for liquefaction while another portion is cooled to a temperature below its critical temperature, so that liquid air can be obtained upon expansion of this cold air stream. This cooled and expanded liquid containing air is then fed into the distillation system for separation. A portion of the isentropically expanded, mainly vapor bearing air can also be fed into the distillation system to supplement the vapor feed necessary for the distillation system. Since all the air, including that fed to the distillation system, enters the cold box at pressures significantly higher than that required by the distillation system, the feed air is used for refrigeration generation or condensation before it enters the distillation system. As compared to the nitrogen recycle liquefiers, this reduces the recirculation flow needed for generating the desired refrigeration which translates into (1) less power loss due to pressure drop, (2) less energy degradation due to heat transfer of the recycle streams and (3) less heat exchanger area. A problem with the air recycle liquefier however, is that as the liquid demand (as a percentage of feed air) increases, the fraction of liquid air in the total feed air increases. This will have an adverse effect on the distillation operation since a large fraction of liquid air in the feed air means a reduced vapor flow to the distillation system, so that not enough vapor is rising in the higher pressure column to generate the boilup for the lower pressure column and to generate the liquid nitrogen which is demanded as reflux and as product. This problem can be overcome by vaporizing a portion or all of the liquid air (or some other liquid process stream) via heat exchange against a condensing stream of high pressure nitrogen as taught in U.S. Pat. No. 4,705,548. This, however, introduces an extra step, namely condensation of nitrogen and vaporization of the liquid air. Since pressure drops as well as energy degradation are involved in this condensation/vaporization step, it means extra power consumption as well as an extra heat exchanger for the condensation/vaporization.
It is an object of the present invention to improve the energy efficiency of the conventional air recycle liquefier by overcoming the above described problem.